The present invention relates generally to temperature sensing devices and more particularly to those utilizing thermojunction sensing elements.
Sensors of the thermojunction type are frequently employed in a clinical environment to monitor body temperature, and it is preferable for reasons of health, sanitation and hygiene that the sensor be disposable after each use. This necessitates the use of some sort of electrical connectors to interconnect the sensor and an instrument responsive to the voltage developed by the thermojunction to provide a temperature readout. To facilitate this interconnection quick-disconnect connectors having, for example, male and female connector components, is typically used. The male connector is usually associated with the sensing element and the female component with the instrument.
In one prior art system utilizing such a quick-disconnect type connection, the sensor comprises two separately plastic-jacketed copper and constantan wires joined at one of their ends to form a thermojunction. The other ends of the wires are encased in a plastic tubular sheath which is flattened and fused to serve as the male connector of the connection. Bared terminal portions of the wires exit opposite ends of the flattened sheath and are wrapped around the sheath to provide contacts. The system utilizes a female connector component which has a pair of hinged spring-loaded jaws of insulating material carrying copper and constantan contacts which are respectively engageable with the bare copper and constantan wire contacts of the male connector component when the jaws are snapped closed. This arrangement, while generally satisfactory, has had several drawbacks, one being that it is difficult to control the thickness of the flattened plastic tubular sheath. As a result, the sheath at times may be either too thin, which prevents a good electrical connection between the contacts of the male and female connectors, or too thick, which may overstess the jaw spring if the jaws are forced shut.
Reference may also be made to U.S. Pat. Nos. 3,576,516 and 3,416,973 showing temperature sensing devices utilizing thermojunction sensing elements and quick-disconnect type connectors.
Another problem which has arisen in connection with temperature sensors of the type utilizing a thermojunction sensing element is due to the fact that the two conductors used in the element are very small in diameter (typically copper and constantan wires of #36-40 ga.) and thus prone to breakage. This is particularly true of copper wire which has a relatively low tensile strength. In certain prior art systems, the conductors have been separately jacketed in plastic and then twisted to form a two-conductor cable. If this is subjected to a tensile stress only slightly greater than the ultimate tensile strength of the copper wire, the copper wire may break, even though the constantan has a somewhat higher tensile strength, because in the twisted pair cable arrangement there is no way of insuring that the stress will be shared or that the somewhat weaker copper conductor will not have to carry a disproportionate portion of the stress.